This report presents the geometric quality results recorded for the five samples of the WorldView-2 Basic and Standard (processing level 1A and 2A) imagery acquired over the JRC Maussane Test Site.
In order to quantify the influence of ground control points and the incidence angle on orthoimage geolocation accuracy, several configurations of well-distributed GCPs (of 0.1m<RMSEx,y,z<0.6m) were set up, and the WV2 images characterised by various off-nadir angles were tested. Accuracy investigations were performed using different mathematical models of the WV2 sensor were tested, i.e. RPC-based and rigorous models implemented into PCI Geomatics, Keystone Spacemetric, and ERDAS IMAGINE& LPS software based on the accurate DEM of 2m grid and RMSEz=0.6m.
The horizontal accuracy assessment included computing the Root Mean Square Error (RMSE) of the residuals between the positions measured on the final product (orthoimage) and the reference positions of the 12 well-distributed independent Check Points (of RMSEx,y,z<0.6m) for each horizontal component (East, North).
The results lead to the following main conclusions:
The one-dimensional RMS error based on the manual measurement of a set of well-distributed Independent Check Points (ICPs) on the WV2 orthoimage:
• is sensitive to the number of GCPs and decreases with increasing number of GCPs (fig. 9,10);
• increases with increasing off-nadir angle and can reach the value of 2.5 WV-2 pixels for 30°(or higher) off-nadir angle provided 4-9 well-distributed GCPs with mean RMSEx,y < 0.6m and a DTM with 0.6m vertical accuracy is used (fig. 15, 16, 17, 18);
• falls within the CwRS prime sensor accuracy criteria, i.e. an absolute 1-D RMSE of < 2.5m, based on 4 (or more) well-distributed GCPs with mean RMSEx,y < 0.6m, provided a DTM with 0.6m vertical accuracy used.
The report recommends that at least 6 (or 9 in areas with terrain slope diversity and/or high off-nadir angle) well-distributed ground control points during the WV2 sensor modelling phase using the Rational Functions mathematical model should be used. The rigorous model requires a minimum 9 or more ground control points. Accuracy requirements for auxiliary date, i.e. ground control points and DEM, given in ‘Guidelines for Best Practice and Quality Checking of Ortho Imagery’ should be strictly followed.